private void MainWindow_Loaded(object sender, RoutedEventArgs e)
{
    this.viewModel.OpenSampleCommand.Execute(null);
    radDocking1.ToolWindowCreated += RadDocking1_ToolWindowCreated;
}

private void RadDocking1_ToolWindowCreated(object sender, Telerik.Windows.Controls.Docking.ElementCreatedEventArgs e)
{
    var window = e.CreatedElement as ToolWindow;
    if (window != null)
    {
        window.WindowStateChanged += Window_WindowStateChanged;

    }
}

private void Window_WindowStateChanged(object sender, EventArgs e)
{
    var window = sender as ToolWindow;

    var spreadsheet = window.ChildrenOfType<RadSpreadsheet>().FirstOrDefault();
    if (spreadsheet != null)
    {
        spreadsheet.InvalidateVisual();
    }
}